Separation, purification and analysis of metal ions is of great importance in many diverse areas. Examples are hydrometallurgy and electroplating processes, metal recovery and waste treatment, the preparation of high purity materials for the electronic and laser industry and the analysis of trace metals in body fluids. Two of the methods that are inherently applicable to most of these fields are solvent extraction.sup.1 and membrane technologies.sup.2. Solvent extraction has extensively been applied for the separation of metal ions in bulk quantities in hydrometallurgic processes. Over the years, much emphasis has been placed on improving the performance of a very small number of simple ligands by carefully modifying extraction conditions such as pH, masking agents, stripping agents and the nature of the organic solvent. Only marginal efforts have been devoted to the design of tailor-made ligands that would extract a specific metal ion in the presence of a plurality of other.sup.3-6. The object of the present invention is to provide binders that selectively extract a specific metal ion from a mixture of many cations by forming stable complexes of 1:1 stoichiometry. an example is a tetradentate ligand that separates quantitatively Cu.sup.2+ from a mixture of six bivalent transition and related metal ions.